1. Field of the Invention
The present invention relates to a liquid crystal display apparatus. More particularly, the present invention relates to a liquid crystal display apparatus of favorable display quality with no afterimages.
2. Discussion of the Background
Liquid crystal display apparatuses are being widely used in watches or electronic desk calculators due to their properties of being, for instance, thin-sized, light-weighted and of consuming low electricity. Especially twisted nematic (hereinafter referred to as "TN") liquid crystal display apparatuses which perform active driving through, for instance, thin film transistors (hereinafter referred to as "TFT") are employed for use as display devices of word processors or personal computers as well as of televisions, and they are gradually replacing CRTs (cathode ray tubes) which had been most generally used as display devices so far.
However, such a TN liquid crystal display apparatus generally presents drawbacks in that its visual angle is narrow, in that the contrast is degraded when seen from oblique directions, and in that gray scale are observed in such a state as to be reversed. Accompanying the large-sizing of liquid crystal display devices in these days, it is especially desired for wider visual angles since differences in display occur even in the case of observing the lateral and vertical ends of the display surface from the same observing point.
While display is performed in a TN liquid crystal display device by utilizing the property of liquid crystal molecules which are aligned owing to electric field, anisotropy in aligning directions is considered to cause degradations in display quality when seen from oblique directions. That is, in case liquid crystal molecules which are aligned from a single direction are looked at from various directions, their optical contributions are changed so that variations in display occur depending on visual angles. This may be considered to be an essential problem of TN liquid crystal display devices.
For coping with this problem, it has been provided for an in-plane switching liquid crystal display device an example of which is disclosed, for instance, in Japanese Examined Patent Publication No. 21907/1988 or International Publication No. WO 91/10936. This in-plane switching liquid crystal display device is arranged in such a manner that a pair of comb-like shaped electrodes are formed on a single substrate, an electric field is generated to be parallel to the surface of the substrate, and liquid crystal molecules are made to be switched through changes in this electric field. Since the liquid crystal molecules rotate in a parallel manner with respect to the substrate surface along the electric field, the angle with respect to the substrate surface is small. Therefore, changes in display depending on visual angles hardly occur when the liquid crystal molecules are looked at from various directions. Such an in-plane switching liquid crystal display device is manufactured by overlapping the substrate with the comb-like shaped electrodes with a counter substrate, connecting these through an adhesive, injecting a liquid crystal material between these two substrates and adhering a polarizer thereon.
FIG. 15 is a sectional explanatory view of a cell of an in-plane switching liquid crystal display device which array substrate is not conductive. In FIG. 15, numerals 1a and 1b denote alignment layers, numeral 2 a color filter protecting layer, numeral 3 a color filter, numeral 4 comb-like electrodes, numeral 6 a polarizer, numeral 7a and 7b glass substrates, numeral 8 a counter substrate, numeral 10a and 10b regions electrified with electric charge, and numeral 11 a crystal liquid layer. Numerals 10a and 10b denote regions in which electric charge is partially distributed. For instance, in case region 10a is electrified to be (+), region 10b is electrified to be (-). The amount of electrification is identical in positive and negative. In case electric charge is applied through static electricity generated, for instance, through rubbing the surface of the liquid crystal display device, electric charge on the counter substrate is neither diffused nor eliminated to outside the substrate since the counter substrate is not conductive, so that regions are generated in which electric charge is partially stored as shown at 10a and 10b. Such a partial storage of electric charge causes electric discharge on the substrate which in turn lead to damages or destruction of substrates or TFTs. Thus, there are known examples in which conductive films are provided in conventional TN liquid crystal display devices for preventing storage of electric charge and in which any of their components is made to be conductive (see Japanese Unexamined Patent Publication No. 60512/1992, Japanese Unexamined Patent Publication No. 245220/1992, Japanese Unexamined Patent Publication No. 263123/1985, Japanese Unexamined Patent Publication No. 235919/1991, Japanese Unexamined Patent Publication No. 56625/1988 or Japanese Unexamined Patent Publication No. 232459/1993). However, all of these conventional liquid crystal display devices differ from in-plane switching liquid crystal display devices, for instance, in their arrangements of electrodes formed on the substrates, and since they do not employ a driving method in which liquid crystal molecules are made to be switched by utilizing an electric field which is parallel to the substrate surface, they are not effective in achieving the above discussed points of expanding the visual angle and decreasing changes in display depending on visual angles.
The significance of eliminating partial storage of electric charge in the present invention lies in the point that it has made it possible to realize a significantly improved in-plane switching type liquid crystal display device in which a visual angle could be remarkably enlarged.
The relationship between partial storage of electric charge and movements of liquid crystal molecules will now be explained. In an in-plane switching liquid crystal display device, partial storage of electric charge causes the presence of portions on which electric charge is being stored and portions on which it is not. Differences in aligning angles of liquid crystal molecules are generated on portions on which electric charge is being stored and portions on which it is not. It is presumed that this phenomenon causes recognition of afterimage and remarkably degrades the display quality.
In an in-plane switching liquid crystal display device, a pair of comb-like shaped electrodes are formed on one substrate, and the other substrate (hereinafter referred to as "counter substrate") does not have a conductive portion. Thus, in the case electric charge is applied from the exterior through, for instance, static electricity, electric charge is partially stored on the counter substrate without being diffused nor without being eliminated to outside the substrate. That is, there are generated portions on which electric charge is being stored and portions on which it is not. Differences in rising angles of liquid crystal molecules are generated on portions on which electric charge is being stored and portions on which it is not. This phenomenon causes problems that afterimages are recognized and the display quality is remarkably degraded. The present invention has been made to solve these problems, and it is an object thereof to provide a liquid crystal display device on which no electric charge is partially stored and on which no afterimages are generated.